Single-Cell Li+ Charger With A Limited Current AC Adapter

The MAX1879 Li+ Charger

The MAX1879 battery charger for a single-cell Li+ comes in an 8-pin ÁMAX
package. This device, in conjunction with the AC linear transformer adapter
and a PMOS FET, allows safe and fast charging of a single Li+ cell. The
MAX1879 is not only an inductorless required solution, but also the lowest
power dissipated solution among single-cell Li+ battery chargers. Figure
5 shows an actual application circuit, which includes a charging state
LED, a thermistor, and a reverse current protection diode.

Figure 5. The MAX1879 high efficient Li+ charger.

The MAX1879 initiates charging in one of three ways: battery insertion,
charger power-up, or external manipulation of the THERM pin. CHG\ LED turns
off when the on/off duty cycle falls to 1/8, and charging terminates when
the on-chip counter times out. Key safety features include continuous
voltage and temperature monitoring, a programmable charger time-out, and a
8mA precharge current mode to charge near-dead cells. Automatic detection of
input power removal shuts down the device, minimizing current drain from the
battery. An overall system accuracy of 0.75% ensures that the cell capacity
is fully utilized without cycle life degradation.

Figure 6. The output waveform of the 3.7V at 350mA AC adapter and the
charging current with the MAX1879 charger shown in Figure 5.